Freescale Semiconductor HCS08 Series, Reference Manual

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Chapter 2 Pins and Connections
MC9S08JS16 MCU Series Reference Manual, Rev. 4
24 Freescale Semiconductor
2.3.1 Power (V
DD
, V
SS
, V
SSOSC
, V
USB33
)
V
DD
and V
SS
are the primary power supply pins for the MCU. This voltage source supplies power to all
I/O buffer circuitry and to an internal voltage regulator. The internal voltage regulator provides regulated
lower-voltage source to the CPU and other internal circuitry of the MCU.
Typically, application systems have two separate capacitors across the power pins. In this case, there must
be a bulk electrolytic capacitor, such as a 10
μ
F tantalum capacitor, to provide bulk charge storage for the
overall system and a 0.1
μ
F ceramic bypass capacitor located as near to the paired V
DD
and V
SS
power
pins as practical to suppress high-frequency noise. The MC9S08JS16 has a V
SSOSC
pin. This pin must be
connected to the system ground plane or to the primary V
SS
pin through a low-impedance connection.
V
USB33
is connected to the internal USB 3.3 V regulator. See Chapter 15, “Universal Serial Bus Device
Controller (S08USBV1) ,” for a complete description of the V
USB33
power pin.
2.3.2 Oscillator (XTAL, EXTAL)
Immediately after reset, the MCU uses an internally generated clock provided by the multi-purpose clock
generator (MCG) module. For more information on the MCG, see
Chapter 9, “Multi-Purpose Clock
Generator (S08MCGV1) .”
The oscillator (XOSC) in this MCU is a Pierce oscillator that can accommodate a crystal or ceramic
resonator. Rather than a crystal or ceramic resonator, an external oscillator can be connected to the EXTAL
input pin.
R
S
(when used) and R
F
must be low-inductance resistors such as carbon composition resistors.
Wire-wound resistors, and some metal film resistors, have too much inductance. C1 and C2 normally are
high-quality ceramic capacitors that are specifically designed for high-frequency applications.
R
F
is used to provide a bias path to keep the EXTAL input in its linear range during crystal startup; its value
is not generally critical. Typical systems use 1 M
Ω
to 10 M
Ω
. Higher values are sensitive to humidity and
lower values reduce gain and (in extreme cases) could prevent startup.
C1 and C2 are typically in the 5 pF to 25 pF range and are chosen to match the requirements of a specific
crystal or resonator. Be sure to take into account printed circuit board (PCB) capacitance and MCU pin
capacitance when selecting C1 and C2. The crystal manufacturer typically specifies a load capacitance that
is the series combination of C1 and C2 (which are usually the same size). As a first-order approximation,
use 10 pF as an estimate of combined pin and PCB capacitance for each oscillator pin (EXTAL and
XTAL).
2.3.3 RESET Pin
After a power-on reset (POR), the PTB1/RESET pin defaults to a general-purpose input port pin, PTB1.
Setting RSTPE in SOPT1 configures the pin to be the RESET pin containing an internal pullup device.
After configured as RESET, the pin remains RESET until the next LVD or POR. The RESET pin when
enabled can be used to reset the MCU from an external source when the pin is driven low.